Electroluminescence



July 31, 1962 s. v. EDENS ETAL 3,047,762

ELECTROLUMINESCENCE Filed Sept. 2'7, 1960 i Y Y Y 200 300 400 soo e00700 800 900 I000 n00 I200 7 EXCITATION FREQUENCY, CYCLES PER SECONDLUMEN PER VOLT- AMPERE EFFICIENCY PER 0.5 DEGREE SQLID ANGLE POTENTIALVOLTS 4 5 IO ll TIME, MILLISECONDS so s2 s5 SAMUEL V. EDENS CLARENCE I.GOODRICH, JR.

wE LEY H. SEALS BY ATTORN EY nnnauwannnwvluvm United States PatentOfiice 3,047,7fi2 Patented July 31, 1962 3,047,762 ELECTRQLUMlNESCENCESamuel V. Edens, Blacklick, and Clarence I. Goodrich, In, and Wesley H.Seals, Columbus, @hio, assignors to North American Aviation, inc.

Filed Sept. 27, 196i), Ser. No. 58,729 19 Claims. (Cl. 313-108) Thisinvention concerns electroluminescence excitation and particularlyrelates to a method and apparatus arrangements for exciting lamps andother such devices having electroluminescent materials to especiallyobtain an improved elficiency in the conversion of electrical energy tolight.

We have discovered that the electrical energy to light conversionefficiency typically associated with electroluminescent lamps and thelike may be significantly increased if a form of excitation is utilizedwherein the voltage characteristic of the applied electrical energy isessentially that of repeated, periodic, spaced-apart energy pulses ofpreferred voltage shape. More particularly, we advocate thatelectroluminescent lamps and the like be excited by an electrical energysource that continuously generates repeated, time-spaced energy pulseswhich each have a near-instantaneous rise time from a reference voltagevalue to a pre-selected peak voltage value, which each have a zero timeduration :at such peak voltage value, and which each have a subsequentrapid voltage fall off from such peak voltage value and a comparativelyshort fall time to the initial reference voltage value. The timewidth ofeach periodically repeated energy pulse, measured near or along anabscissa line positioned close to the reference voltage level, isgenerally a small fraction of the time period determined by theestablished pulse frequency. Numerous advantages and unobvious resultsare obtained in connection with the practice of our invention.

An important object of this invention is to provide a method andapparatus for exciting electroltnninescent lamps and the like to developtherein an increased effioiency in the conversion of electrical energyto light in comparison to known methods and forms of electroluminescenceexcitation.

Another important object of this invention is to provide a method andapparatus for exciting electroluminescent lamps and the like in a mannerwhich establishes an output light brightness change that is a directfunction of a corresponding input voltage change and which has astraight line relation over a substantial lamp brightness range.

Another object of this invention is to provide a method and apparatusfor combination with known electroluminescent lamps and the like tooperate such electroluminescent devices at increased output lightbrightness when compared to the brightness obtained using conventionalmethods and apparatus at a similar operating frequency and at acorresponding peak-to-peak operating voltage value.

Another object of our invention is to provide a method and apparatus forelectroluminescence excitation which can be effectively applied toelectroluminescent lamps and the like to permit the use of comparativelyhigher pealoto-peak operating voltage values for improved light outputwithout establishing an increase in the level of danger of possibleelectrical shock.

Still another object of this invention is to provide a method andapparatus for exciting electroluminescent lamps to develop :a linearoutput brightness change which is a straight line function of acorresponding pulse frequency change throughout a substantial pulsefrequency range.

Another object of our invention is to provide a method and apparatus forelectroluminescence excitation which may be effectively utilized tooptimize the lumen per watt efficiency for an electroluminescent devicewithin given peak-to-peak operating voltage and pulse frequency values.

Other objects and advantages will become apparent during considerationof the description and drawing portion of this application.

In the drawings:

FIG. 1 is a schematic and sectional illustration of an apparatusarrangement which may be utilized in the practice of this invention;

FIG. 2 graphically illustrates the voltage characteristics which arepreferably developed in connection with this invention;

FIG. 3 provides information regarding the electrical energy to lightconversion efficiency typically associated with this invention andregarding electrical energy to light conversion etliciencies associatedwith known methods and conventional apparatus for electroluminescenceexcitatic-n;

FIG. 4 schematically illustrates a portion of an alternate apparatusarrangement which may be used in connection with our invention; and

FIG. 5 schematically illustrates an alternate power supply apparatusarrangement which may be used in practicing our invention.

A preferred embodiment of the apparatus of our invention is illustratedschematically :and sectionally FIG. 1 and essentially consists of apower supply connected to the conventional electroluminescent lampdesignated 10. Lamp ltl is provided with metallic back-up foil 11serving as one conductive layer, transparent material 12 serving asanother conductive layer, intermediate layer 13 of electroluminescentmaterial such as a suitable phosphor, and an overlay face layer 14 oftransparent glass. Such component layers of lamp in are fabricated usingknown techniques and compositions of material. More specifically, lamp10 may be laminated with a foil 11 fabricated of aluminum, copper, orthe like, a transparent material 12 comprised of stannous chloride, anda layer 13 which is a phosphor such as a zinc sulfide derivative that iscontained or dispersed in a non-conductive ceramic. Also, the form ofelectroluminescent lamp illustrated in FIG. 1 derives its structuralintegrity essentially through the basic rigidity of the overlay glassface designated 14.

The form of power supply illustrated in FIG. 1 in electrically-connectedrelation to lamp 10 at contact connections 15 and 16 is important inthat the objects of our invention are principally obtained thereby. Suchpower supply is constructed to include a source of direct-currentelectrical energy (battery) 17, circuit components 18 through 20 forconveniently adjusting or varying the magnitude of the output voltageassociated with the energy derived from source 17, circuit components 21through 28 for developing electrical energy that voltage-wise ischaracterized by waves that are repeated at a pre-selected frequency andthat each have a square form, and circuit components 29 through 32 forconverting the energy developed across winding 21 into energy pulses ofthe preferred voltage-time shape. Components 21 and 29 comprise theprimary and secondary windings of a transformer device having core 32.Core 32 is preferably characterized as having a rectangular hysteresisloop so that windings 21 and 29 will exhibit low inductance atsaturation.

Circuit components 18 through 20 are essentially provided forconvenience in varying the output frequency and voltage of the powersupply. The particular square-wave output frequency developed at winding29 is proportional to the input voltage in accordance with the relationwhere V is the direct current potential measured across terminals 33 and34, K is a constant having a value of 4.0, A is the cross-sectional areaof core 32, N is the number of conducting turns in primary winding 21,and B is the flux density of core 32. In the embodiment illustrated inFIG. 1, input voltage variation is achieved through the use oftransistor 18, fixed resistor 19, and variable low-power resistor 20.The load for practicing this invention is taken from the energy sourceacross terminal points 33 and 34. From a careful consideration ofcomponents 17 through 20, it will be noted that a limited current flowis provided in the load circuit and input voltage variation is achievedthrough use of low power variable resistor 20. The rheostat 39 shown inFIG. 4 may be considered as performing a function similar to thatachieved by components 18 through 20. As previously noted, FIG. 4provides a partial schematic illustration of apparatus which may be usedin connection with this invention but essentially details only analternate form of input voltage control means, Use of a voltagevariation means such as circuit grouping 18 through 20 or rheostat 39 isnot mandatory to the practice of our invention. For instance, theapparatus arrangement of FIG. does not employ a voltage variationdevice.

One side of the square-wave-forming circuit is established at connection34 which is a center tap with respect to taps 35 and 36. The other sideof such circuit group is connected by connection 33 to the negative sideof energy source 17. This latter connection leads to the collectors oftransistors 22 and 25. Transistor components 22 and 25 are preferablyselected for highfrequency operation and are characterized as having avery high gain to limit the heat otherwise dissipated by resistors 23and 26. Such resistors are provided for the purpose of limiting basecurrent in their respective associated transistor device.

A degree of kick-back voltage is desierd in Winding 21 to operate thetransistors and achieve an oscillatory action. Because of theabove-stated low winding inductance at transformer core saturation,excessive kick-back voltage transients are avoided to thereby minimizethe possibility of transistor damage. As a precaution, diodes 24.- and27 are added to the square-wave-forming circuitry to further prevent thepossibility of voltage kick-back transients damaging transistors 22 and25 Diode 24 and 27 are connected to winding 21 at connections 35 and 36,respectively, and to the negative side of source 17 through connections37 and 38, respectively. Resistor 23 is provided to initiate theoscillating switching action of components 22 and 25. It is essentiallya bias resistor for the base of transistor 25. When the components ofcircuit grouping 21 through 23 are subjected to source 17, transistor 25conducts first. Components 22 through 28, in combination with core 32,provide a high overall switching efficiency. The resulting voltage formdeveloped across winding 29 should be as ideal a square- Wave aspossible to thereby permit the formation of highly reproducible shapedpulses by components 30 and 31 That portion of the apparatus illustratedin FIG. 1 as being comprised of components 30 and 31 performs adifferentiating function and is provided for developing pulses ofelectrical energy having a proper wave (voltagetime) shape. Resistor 30and capacitor 31 are combined in an RC circuit to develop acomparatively short time constant for the shaping circuit, and areeifective to impart the preferred wave shape to the electrical energy ofsquare-wave voltage form transmitted to winding 29. Additional detailsare provided hereinafter with respect to the selection and sizing ofcomponents 30 and 31 to develop the preferred shaped voltage pulseutilized in this invention.

One particular electrical energy output developed by the apparatusarrangement of FlC 1 and having the voltage-time pulse shape which ispreferred in connection with he practice of this invention isillustrated in FIG. 2. Electrical energy pulses through 49 are shown inFIG. 2 in the manner of an oscilloscope screen presentation, and aredeveloped across connections and 16 of the equipment of FlG. l, have apeak-to-peak voltage range which extends from 300 volts positive to 300volts negative, and have a frequency of 400 cycles per second. outputpulse frequency is considered to be 400 (cycles) per second although 800separate pulses, positive and negative, are developed each second. Inthe illus trated electrical energy wave form of PEG. 2 each pulsedevelops a peak voltage value (positive or negative) from a referencevoltage of zero level with a near-instantaneous i 3 time, has anear-zero time duration at the maximum voltage value, and returns to thereference voltage level (zero) with a rapid voltage fall-off and with acomparatively short fall time. The rise of each pulse to its maxivoltagevalue appears as a near-vertical presentation and as shown in HS. 2 thepulse width, as measured at abscissa level near the reference voltagevalue, comprises only a small portion of the base time periodestablished by the output pulse frequency. Referring to FIG. 2, thetypical pulse width measured at essentially the reference voltage valueof zero volts is approximately 0.3 millisecond and constitutes aboutone-eighth /8) of the base time period of 2.5 milliseconds. The RCcircuit components and 31 used to develop pulses through 49 included aresistor 30 having a value of 10K ohms and a capacitor 31 having a valueof 0.01 microfarad. The specific resistance and capacitance valuesstated in this application have reference to an apparatus arrangementwhich employs an electroluminescent lamp having a lightemitting surfacearea of approximately 20 square inches. Sucn specific values will changein magnitude when similar electrical energy pulses having a shapedvoltage form are developed for electroluminescent lamps of differentoutput area.

The significance of the instant invention is illustrated in H6. 3wherein a graphical comparison is made of electrical energy to lightconversion eificiencies associated with a typical electroluminescentpanel lamp excited by conventional techniques in relation to conversioneificien cies associated wtih the practice of our invention. Theefficiency values plotted in FIG. 3 were obtained using the apparatusarrangement of FIG. 1. The electroluminescent panel lamp ltl provided inthe arrangement was a commercially-available electroluminescent panellamp which emitted green light.

Curve 50 is established from information obtained through themeasurement of lamp brightness developed by applied, repeated, shapedvoltage pulses having a constant pealr-to-peak voltage and a constantpulse width but at different output frequencies over a range whichextends from 200 cycles per second to 1200 cycles per second (400individual pulses per second to 2400 individual pulses per second). TheRC circuit components utilized to develop shaped voltage pulses having a0.1 millisecond pulse base width included a 0.001 microfarad capacitor31 and a 10K ohm resistor 30. Peak-to-peak voltage was maintained at 600volts (+300 volts to -300 volts) and the root-mean-square voltagemeasured with a non-true R.M.S. meter at different excitationfrequencies varied from volts at 1200 c.p.s. (cycles per second) to 42.5volts at 800 c.p.s. to 30 volts at 500 c.p.s., to 24 volts at 400c.p.s., and 13 volts at 200 c.p.s. Lumen per voltampere efficiency(based on a 0.5 degree solid angle of emitted light) varied fromapproximately 0.06 at 1200 c.p.s. to 0.09 at 800 c.p.s., to 0.12 at 500c.p.s., to 0.17 at 400 c.p.s., and to 0.30 at 200 c.p.s. True R.M.S.voltage values for a particular curve should remain constant throughoutits entire frequency range. This is for the reason that R.M.S. voltagein all cases is a function that is independent of a time value as such.

Curve 51 is comparable to curve 50 except that the pulse width of eachrepeated shaped voltage pulse was varied to a constant 0.2 millisecondvalue at its base through the use of RC circuit components whichincluded a 0.0 05 microfarad capacitor 31 and a K ohm resistor 30. Asimilar 600 volt peak-to-peak voltage was maintained and the frequencyvaried from 200 cycles per second to 1200 cycles per second. Measuredroot-meansquare voltage values for observations related to curve 51varied from 75.0 volts at 1200 c.p.s., to 55.0 volts at 800 c.p.s., to39.5 volts at 500 c.p.s., to 33.0 volts at 400 c.p.s., to 18.5 volts at200 c.p.s. Efiiciency values for curve 51, in terms of the ordinateunits of FIG. 3, varied from 0.07 at 1200 c.p.s., to 0.105 at 800c.p.s., to 0.14 at 500 c.p.s., to 0.165 at 400 c.p.s., to 0.28 at 200c.p.s. It should be noted that the excitation developed in connectionwith curve 51 provides a generally better measured efiiciency than curve50 throughout the range of from approximately 400 cycles per second andup.

Curve 52, which also is based upon the features of our invention, wasdeveloped from measurements made in connection with the excitation ofthe hereinbefo-re-identified electroluminescent panel lamp usingrepeated shaped voltage pulses having an individual pulse width of 0.3millisecond. Peak-to-peak voltage was maintained at a level of 600 voltsand pulse frequency, in cycles per second, was varied over the graphedrange. Measured root-mean-square voltage values varied from 100 volts at1200 c.p.s., to 72 volts at 800 c.p.s., to 52 volts at 500 c.p.s., to 44volts at 400 c.p.s., and to 24 volts at 200 c.p.s. Lumen per volt-ampereefficiency was determined to vary from approximately 0.065 at 1200c.p.s., to 0.10 at 800 c.p.s., to 0.12 at 500 c.p.s., to 0.14 at 400c.p.s., and to 0.15 at 200 c.p.s. The relative efficiency is slightlybetter than that of curve 50 at frequencies above 500 c.p.s but notquite as good as the efficiency associated with the 0.02 millisecondpulse excitation of curve 51 in the same frequency range. The individualpulse base width, as a percentage of pulse frequency time period, can becomputed using a conventional approach. By way of example, the pulsewidth percentage at 1200 cycles per second varies from 12% to 36% for0.1 and 0.3 millisecond pulses, respectively, to percentages of 2% and6% at 200 cycles per second for corresponding pulse widths.

The electrical energy to light conversion efficiencies associated withthis invention, as presented in curves 50 through 52, are especiallyimportant when considered in light of curves 53 through 55 of FIG. 3.Curve 53 graphs the efficiencies obtained by exciting anelectroluminescent lamp with a conventional square-wave voltageexcitation, curve 54 represents the efficiencies obtained through use ofa conventional sine-wave voltage shape excitation, and curve 55 relatesto excitation using a triangular-shaped voltage wave. A 100 voltroot-mean-square voltage value was held constant in developing all theinformation for establishing curves 53 through 55 throughout theindicated frequency range. Peak-to-peak voltage was varied from valuesof 200 volts for curve 53 to 282 volts for curve 54 to 400 volts forcurve 55. The electrical energy to light conversion efficienciesassociated with curves 53 through 55 are significantly lower than theefficiency curves 50 through 52 associated with this invention. In thecase of square-wave excitation, the lumen per Volt-arnpere efiiciencymaintained a. nearly-constant value of approximately 0.01 over thefrequency range from 200 c.p.s. to 1200 c.p.s. The conversion efiiciencydepicted by curve 54 is a nearly-straight line relation from a value ofapproximately 0.025 lumen per volt-ampere at 200 c.p.s. to 0.015 lumenper volt-ampere at 1200 c.p.s. Triangularshaped voltage wave excitationestablished an improved efficiency, but one which varied only throughoutthe range of from 0.025 lumen per volt-ampere at 1200 c.p.s. to 0.05lumen per volt-ampere at 200 c.p.s.

Several general observations should be noted. First, the data developedfor use in FIG. 3 involved brightness measurements for anelectroluminescent panel lamp having a leading power factor. Brightnessmeasurements were made with a conventional brightness meter wherein aparticular color response factor of 3.0 was established for measuringthe green light emitted by the electroluminescent lamp. Volt-amperemeasurements were utilized in developing the values plotted in FIG. 3 asa matter of convenience. The relative efiiciency values of FIG. 3 of thedrawing can be corrected to determine absolute lumens per wattefliciency by applying a correction factor. This correction factor takesinto consideration total light emitted through a hemisphere, thebrightness meter color response factor, and power factor, in accordancewith conventional analyses.

FIG. 5 illustrates an apparatus arrangement which may be utilized toobtain the advantages of our invention in connection with a conventionalalternating current electrical energy source. As illustrated therein,source 60 may be a typical volt, 60 cycle, single-phase source ofelectrical energy such as is commonly used in domestic lightingapplications. Source '60 provides an output having a sinusoidalwave-form as to its voltage-time relation and is connected to winding 63by the circuit lines designated 61 and 62. Winding 65, winding 63, andcore 64 comprise a suitable transformer coupling device for voltageamplification. Circuit components 66 through 73 are provided inaccordance with the teachings of our invention. Capacitor 66 andresistor 67 comprise the differentiating components to develop theherefore-described shaped voltage pulses. A square-wave form electricalenergy output is developed at terminals '68- and 69 by Zener diode 70.The shaped voltage pulses are applied to electroluminescent lamp 71 byconnections through the line components 72 and '73.

As used in this decription, the term shaped voltage pulse refers to anindividual pulse of electrical energy having a voltage-time relationwherein the voltage value has a near-instantaneous rise time from areference voltage to a peak voltage (positive or negative), hasessentially a zero dwell time at the peak voltage value, and has acomparatively rapid voltage fall off and a short fall time from the peakvoltage value to the initial reference voltage value. Such shapedvoltage pulses are also characterized as having a total time duration atvoltages sig nifieantly above the reference voltage level which is lessthan fifty percent (50%) of the time period established by the outputpulse frequency (cycles per second). In the examples given above, pulsewidth varies for 2% of the period established in connection with a 0.1millisecond pulse at 200 c.p.s. to 36% of the period established inconnection with a 0.3 millisecond pulse at 1200 c.p.s. The termnear-instantaneous must be interpreted in light of our observance thatan instantaneous rise time would be present if the electroluminescentlamp component of the apparatus arrangement had infinitely smallresistance rather than finite capacitance.

The improved efficiency achieved with shaped pulse voltage excitation isnot entirely derived from the effected reduced root-mean-square voltage.Although minimum root-mean-square voltage values were achieved inconnection with curve 50 an optimum efficiency appears to have beenobtained in connection with curve 51 relating to a 0.2 millisecond pulsebase width. Greatest lamp brightness, achieved at somewhat reducedefficiency, was developed in connection with curve 52. We have, throughthe developed data, clearly established that increased brightness can beobtained from commercial electroluminescent lamps as a result of theimproved conversion efliciences obtained through practice of ourinvention. Also, higher peak-to-peak voltages at high excitationfrequencies can be applied to commercially-available electroluminescentlamps in connection with the practice of this invention with accidentalcontact to exposed lamp terminals resulting in little or no electricalshock hazard. This advantage is attributed largely to the reduced energy7 available at the peak of each shaped voltage pulse of electricalenergy. In conventional sine-wave excitation root-mean-square voltagevalues increase proportionally to applied peak-to-peak voltage and notas a function of frequency.

We have also observed in connection with our invention that an increasein apparatus input voltage is mam? fested by a corresponding increase inlamp brightness. The observed excellent linearity was in connection withan electroluminescent lamp arrangement corresponding to FIG. 1 andutilized 0.6 millisecond base width repeated shaped voltage pulsesthroughout a range of from 357 cycles per second to 500 cycles persecond and with respective input voltage values extending from 25 voltsDC. to 35 volts D.C. (measured at terminals 33-34). Use of ourelectroluminescent excitation invention, therefore, offers advantages inthat brightnes changes can now be utilized to measure or detect changesin voltage with true straight-line linearity over substantial inputvoltage ranges.

It is to be understood that the forms of the invention herein shown anddescribed are to be taken as preferred embodiments of the same, but thatvarious changes in the shape and size of parts may be resorted toWithout departing from the spirit of the invention or the scope of thesubjoined claims.

We claim:

1. A method of exciting an electroluminescent lamp which includes thesteps of applying successive pulses of electrical energy to said lamp,said pulses each being characterized by a voltage-time history having anear-instan taneous rise time from a reference voltage value to a peakvoltage value, having a zero time duration at said peak voltage value,and having a comparatively rapid voltage fall off from said peak voltagevalue to said reference voltage value.

2. The method defined in claim 1, wherein said electrical energy pulsesare repeated at a pre-selected frequency, the duration of each of saidpulses being substantially less than the time period established by saidpulse repetition frequency.

3. The method defined in claim 1, wherein each of said electrical energypulses is separated from a succeeding electrical energy pulse by afinite period of time at essentially said reference voltage value.

4. A method of exciting an electroluminescent lamp means which includesthe step of applying electrical energy pulses to said lamp means, saidpulses being characterized by a voltage-time history which extends froma reference voltage value alternately to a positive peak voltage valueand to a negative peak voltage value, said voltage-time historyincluding a near-instantaneous rise time from said reference voltagevalue to one of said peak voltage values, a zero time duration thereat,a rapid voltage fall ofi from said one peak voltage value to saidreference voltage value, a near-instantaneous rise time to the other ofsaid peak voltage values, a zero time duration thereat, and a rapidvoltage fall off from said other peak voltage value to said referencevoltage value.

5. The method defined in claim 4, wherein said pulses are repeated at afrequency which establishes a prescribed time period which is uniform asto successive like pulses, said pulses each having a duration which issubstantially less than one-half of said time period.

6. The method defined in claim 4, wherein successive pulses of saidelectrical energy pulses are separated from each other by a finiteperiod of time at essentially said reference voltage value.

7. A method of exciting an electroluminescent lamp which includes thestep of applying successive shaped voltage pulses of electrical energyto said lamp, said shaped voltage pulses each being formed over avoltage range which extends from a reference voltage value to a peakvoltage value, and each said shaped voltage pulse being separated by afinite time interval from its succeeding ii shaped voltage pulse at thelevel of said reference voltage value.

8. A method of exciting an electroluminescent lamp means which includesthe step of developing successive pulses of electrical energy which arecharacterized by a square-wave-voltage form, converting said square-wavevoltage form electrical energy pulses into electrical energy pulseshaving a shaped voltage form, and applying said shaped voltage formelectrical energy pulses to an electroluminescent lamp means, saidshaped voltage form including an instantaneous rise time from areference voltage value to a peak voltage value, zero dwell time at saidpeak volage value, and a rapid voltage fall off from said peak voltagevalue to said reference voltage value.

9. The method defined in claim 8, wherein said squarewave voltage formelectrical energy pulses are repeated at a pre-selected frequency, theduration of each of said shaped voltage form electrical energy pulses atall voltage values other than essentially said reference voltage valuebeing substantially less than one-half the period established by saidpre-selected frequency.

10. The method defined in claim 8, wherein said square wave voltage formelectrical energy pulses are repeated at a preselected frequency, theduration of each of said shaped voltage form electrical energy pulses atother than approximately said reference voltage value being less thanapproximately 36% of the repeated period established by saidpre-selected frequency.

11. A method of converting a variable input voltage into a proportionalvariable light brightness and which includes the steps of: convertingelectrical energy having a potential which is proportional to saidvariable input voltage into spaced-apart successive pulses of electricalenergy, and exciting an electroluminescent material in a lamp by saidpulses to thereby produce a variable light brightness, said pulses eachbeing characterized by a near-instantaneous rise time from a referencevoltage value to a variable peak voltage value, by a zero duration atsaid variable peak voltage value, and by a comparatively rapid fall offof voltage from said variable peak voltage value to said referencevoltage value.

12. The method defined in claim 11, wherein said spaced-apart pulses ofelectrical energy are repeated at a variable frequency when said inputvoltage is varied, said variable frequency being proportional to saidvariable input voltage over a substantial input voltage range.

13. The method defined in claim 11, wherein said spaced-apart pulses ofelectrical energy are repeated at a variable frequency when said inputvoltage is varied, the duration of each said pulse being constant over asubstantial frequency range and being substantially less than onehalfthe period established by each frequency in said frequency range.

14. Apparatus comprising, in combination: a lamp means having anelectroluminescent material which emits visible li ht when excited, apower supply which develops an electrical energy output for excitingsaid electroluminescent material, and conducting means electricallyconnecting said power supply to said lamp means electroluminescentmaterial, said power supply developing an electrical energy output whichincludes successive spacedapart electrical energy pulses characterizedby a nearinstantaneous rise time from a reference voltage value to apeak voltage value, a zero dwell time at said peak voltage value, and acomparatively rapid voltage fall off from said peak voltage value tosaid reference voltage value. 1

15. The apparatus defined in claim 14, wherein said power supplyincludes a resistance-capacitance circuit portion having a short timeconstant, said conducting means being directly connected to said circuitportion and to said lamp means.

16. The apparatus defined in claim 14, wherein said spaced-apart pulsesare repeated at a pro-selected frequency, the duration of each of saidpulses being substantially less than the time period established by saidfrequency.

17. Apparatus comprising, in combination: first means which repeatedlydevelops an electrical energy output having a square-Wave voltage form,difierentiating means connected to said first means and which changeseach square-wave voltage form electrical energy output received fromsaid first means into an electrical energy pulse which has anear-instantaneous rise time from a reference voltage value to a peakvoltage value, zero duration at said peak voltage value, and a rapidvoltage fall off from said peak voltage value to said reference voltagevalue, illuminating means having an electroluminescent materialcontained therein, and separate means connected to said differentiatingmeans and to said illuminating means to conduct each electrical energypulse developed by said differentiating means to said electroluminescentmaterial.

References Cited in the file of this patent UNITED STATES PATENTS r2,843,804 Diemer July 15, 1958 2,895,081 Crownover July 14, 19592,937,298 Putkovich May 17, 1960

